One in four girls, compared to one in eight boys, is likely to warrant a diagnosis of Major Depressive Disorder by the time she finishes adolescence. What exactly causes these disturbing sex differences is at the center of decades of intense research and debate, yielding few definitive answers thus far. Recent support has emerged for Response Style Theory, positing that gender differences in rumination, or the tendency to perseverate about symptoms and causes of one's depression, may explain a large part of the these sex differences (Nolen-Hoeksema, 1994). The mechanisms by which rumination causes this gender disparity, however, are still unknown. This study proposes to test a new moderated mediational model of rumination in middle childhood and early adolescence to help explain the sex differences in prevalence rates of depression during adolescence. Our application examines how female gender identification may mediate the relation between sex and rumination, and how rumination may in turn mediate the link to depressive symptoms. Furthermore, we will look at how this model is moderated by the effects of age and pubertal development. We hypothesize that this mediational model will explain why the sex-depression link only becomes evident during adolescence, but not in childhood. Adding such a moderational component to the model will help explain why pre-pubertal girls and boys have similar rates of depression, but girls become more likely to develop depressive symptoms later on. We will administer questionnaires to 450 students, their parents, and their teachers in grades 4, 6 and 8 at three time points during the school year (approximately 3 months apart). We will use latent variable structural equation modeling (SEM) to test our mediational hypotheses and multi-group SEM to test our moderational hypotheses. By advancing our understanding of the emergent gender difference in depressive symptoms, we will contribute to knowledge about the etiology of depression for both males and females. This knowledge will enable clinicians to target high-risk youth for prevention and intervention programs and will facilitate the distribution of resources to those who need them most. This project dovetails nicely with the priority given to research on gender differences in adolescent depression by the Division of Pediatric Translational Research and Treatment Development.